CPT International 04/2016

Process step
Cavity filled, die closed, solidification
Die open, waiting, ejection of casting
Spraying
Blowing
Die open, waiting for die closing
Die closed, waiting for shot
time
0-10 s
10-20 s
20-22 s
22-24 s
24-30 s
30-40 s
Table 1: Cycle data for the simulation of the heat balance
Heat transfer pair
Heat transfer coefficient in W/m²K
Melt - Die 10,000
Spraying medium - Die 10,000
Blowing air - Die 250
Ambient - Die
temperature dependent
Oil channel - Die 2500
Table 2: Heat transitions in the individual process steps
tion were applied as thermal loads at
each time step of the mechanical simulation.
As only one load cycle was taken
into account the material behaviour
was supposed to be linear elastic. Figure
8 shows the surface stress (parallel to
the die surface) of the die made of D185
at the end of solidification and after
quenching the surface via the spraying
process. Corresponding with the thermal
load of the die surface at the end
of the solidification the surface suffers
compressive stress. After the ejection of
the casting and subsequent spraying of
the die the temperature gradient is reversed.
Because of the increased base
temperature of the die far away from
the surface the reversed temperature
field leads to tensile stresses. At the die
surface the tensile stress has its highest
level which could lead to the initiation
of fire cracks in reality. Figure 9
shows a comparison of the time-stress
curves for the materials 1.2343 and
D185. In figure 9 it can be seen that the
temperature and corresponding stress
peaks are much lower in D185 than in
1.2343. Under real conditions the surface
stresses are overlapped with chemical
reactions between melt and die material.
However these effects have not
been taken into account in this work.
Conclusions
Due to its increased heat conductivity
with respect to steels a better heat removal
and a better resistivity against
thermal shocks may be expected by using
D185 in hpdc-applications. These
expectations were proven in test facilities
and by numerical modelling. The
following conclusions may be drawn:
» The obtained cooling rates for tungsten
compound D185 at the test facility
are higher than that of iron
based materials,
» the calculated solidification time using
tungsten compound D185 is lower
than observed for steel 1.2343,
» the calculated temperature peaks for
D185 are lower than that for steel
1.2343
» due to the decreased temperature
peaks decreased stress peaks may be
expected.
It has to be considered that the direct
prediction of damage initiation and
die life time cannot be derived from
the simulations done in this work. This
is due to the fact that the thermo-mechanical
fatigue and the plastic material
behaviour of the materials were
not taken into account. Nevertheless
a positive effect of the decreased loads
can be expected.
References:
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